Alkaline and acid thermal hydrolysis of biological excess sludge in sequencing batch reactor

Abstract

The efficiency of biological wastewater treatment processes is based on pH sensitivity which leads to the reduction in the amount of sludge production. In order to achieve the best result, thermal hydrolysis is recognized and used, although acid thermal hydrolysis has serious drawbacks (corrosion, required post-neutralization, etc.). Alkaline thermal hydrolysis has been less remarkable, as the subject of the detailed pilot-scale research of two sequencing batch reactors (SBR) reported in this study. Long-term (about 8 months) continuous experiments were conducted to identify the effect of pH increasing from 4 to 11 in controlled temperature of 60°C to achieve the optimum pH for the reduction of excess biological sludge. After providing a steady state in the reactors, sampling and testing parameters including chemical oxygen demand (COD), mixed liquor suspended solids (MLSS), dissolved oxygen (DO), specific oxygen uptake rate (SOUR), sludge volume index (SVI), and yield (Y) coefficient were evaluated. Results showed that in pH of 9, the kinetic yield coefficient decreased from 0.63 to 0.33. Therefore, excess sludge declined to 44%. Moreover, the soluble COD increased slightly in the effluent, whereas the removal percentage decreased to 79 in the reactor while the amount of SVI and SOUR in this pH dropped to 65 mg/l and 12 mgO2/l.g VSS, respectively. However, no sludge was seen in the higher pH or lower pH, whereas effluent soluble chemical oxygen demand (SCOD) and turbidity was increased. Additionally, the wastewater disposal standards were not achieved and had a bad odor.